The present invention relates to a computerized tomographic apparatus for reconstructing a tomogram of an object of interest, thereby diagnosing the object.
Computerized tomography is known as a system for measuring the internal flaws, composition and structure of an object with high precision in a nondestructive manner.
Such computerized tomography uses a radiation source for radiating an X-ray as a flat fan beam expanding in a sector shape. The object to be measured is irradiated with the fan beam generated from the radiation source, and the fan beam is detected by a plurality of radiation sensors arrayed along the expansion direction of the fan beam. The radiation source is sequentially rotated in units of degrees through 180 degrees to 360 degrees while the radiation source is located opposite to the radiation sensors with the object at the center. X-ray absorption data from different directions of a slice of the object is acquired. The acquired data is reconstructed by a computer to form a tomogram. Therefore, image reconstruction can be performed with about 2,000 gradation levels at the respective positions in accordance with the composition, and the state of the slice can be examined in detail.
Computerized tomography of this type is called a 3rd generation system. In a 1st generation system, a radiation sensor is located opposite to a radiation source for radiating an X-ray as a pencil beam. The radiation source and sensor are traverse scanned along the slice of the object. The object or radiation source is rotated by a predetermined angle for every traverse scanning.
In a 2nd generation system, an X-ray as a fan beam with a narrow width and a plurality of radiation sensors are used. The radiation source and sensors are subjected to traverse and rotary scanning.
In a 4th generation system, a plurality of sensors are arranged around the object to be examined and are combined with a radiation source for radiating an X-ray as a fan beam with a large width. Only the radiation source is rotated.
Reconstruction process of the computerized tomography is typically classified into an analytic technique and an algebraic technique. Among these techniques, the analytic technique, especially filtered back projection is mainly used. According to filtered back projection, radiation intensity data is convoluted by using a filter function, and the resultant projection data is back projected to perform image reconstruction. Although such filtered back projection is very effective, noise is emphasized since the high-frequency component of the projection data is emphasized to improve resolution of the image. Assuming that noise included in the projection data is limited to quantum noise, an S/N ratio is decreased when a dose of radiation transmitted through the object is small. The image is thus degraded. This phenomenon also occurs when radiation is transmitted through a material with high absorbancy of radiation within a circular region of interest which is defined by the outermost edge of the X-ray during rotary scanning, and the dose of this radiation is very small.
When the dose of the source is small or the dose is locally decreased, the conventional filtered back projection method is not suitable for reconstructing the tomogram in accordance with the projection data.
Furthermore, the conventional filtered back projection method has a disadvantage in generation of artifacts as compared with approximation method. This disadvantage is typically observed when the number of projection data is small although an image to be reconstructed is complicated.